The present invention relates to an arrangement and bracket for holding a sensor such as a magnetic sensor in a fixed position with respect to a target.
Magnetic sensors operate on the principle of detecting magnetic flux density modulation caused by the movement of an appropriately configured target (or targets). The magnetic sensor must be affixed very close to the target since its sensitivity decreases very rapidly with the size of the air gap between the target and the magnetic sensor. In most automotive applications, for example, the air gaps are on the order of 0.3 to 1.75 mm. Over such a range of air gaps, the sensor output signal decreases more than ten times. The signal attenuation at large air gaps makes the sensor operation more prone to noise induced failures as well as less accurate in detecting the elements of the target as it spins in relation to the magnetic sensor. Both of these factors are often unacceptable in critical engine control and diagnostic applications.
However, in the majority of production cases, the stack-up of tolerances of the many different components randomly influence the net size of the air gap, which consequently precludes achieving, at each assembly, a precisely predetermined air gap by mere assembly of the parts. As a result, because of the random variations caused by accumulation of tolerances, mere assembly of the parts risks damaging interference between the magnetic sensor and target on the one hand, and inaccurate readings associated with too large an air gap on the other hand. To lessen all the tolerances so that mere assembly assures, at each assembly, the optimum air gap is physically unrealistic and involves inordinate costs associated with manufacturing such precise parts.
The majority of magnetic sensors used in automotive applications involve non-adjustable air gap placement, wherein the stack-up of tolerances causes deviation from the optimal air gap. For example, a rigid bracket is affixed to the body of a magnetic sensor. The magnetic sensor is placed into a sensor port in the engine block, and the bracket is bolted, via a bolt hole in the bracket, to a threaded mounting hole in a mounting surface of the engine block. When the bracket is bolted, the length of the sensor body from the sensor port surface to the sensor tip determines the air gap with respect to the target, which air gap is affected by the stack-up of tolerances. Even though subject to tolerance related placement inaccuracy, this structural mounting methodology is used widely because of the simplicity of the hardware, and ease of assembly and service.
In situations where air gap variation cannot be tolerated, the air gap is preset during magnetic sensor installation by means of an adjustable bracket, often referred to as a xe2x80x9cside mountxe2x80x9d bracket. The adjustability of side mount brackets resides in a bolt slot which allows for the bracket to be adjusted along the slot elongation relative to the threaded mounting hole of the mounting surface.
In one form of operation of a side mount bracket, the sensor body is placed into the sensor port of the engine block such that the sensor tip is allowed to touch the surface of the target, and then it is withdrawn a distance equal to the predetermined optimum air gap. This method is more time consuming and is error prone.
In the prior art, it is known to precisely adjust the air gap using a threaded sensor body and threaded sensor port. This structure is generally used exclusively with magnetic sensors having a single sensing element and having sensing capability unaffected by sensor rotation around its longitudinal axis. In this approach, the sensor tip is brought into touching engagement with the target, and then the sensor body is rotated a predetermined angular amount, wherein the pitch angle of the threads raises the tip a distance equal to the optimum air gap. However, most automotive magnetic sensors contain more than one sensing element and are designed to operate at only one particular angular setting around the sensor axis. Consequently, a threaded sensor body would need to be adjusted in whole revolution steps (i.e., 360 degrees) and air gap adjustment would then be in steps of the thread pitch. While the use of a sufficiently small pitch may render the air gap setting resolution adequate, many sensors are precluded from rotation due to geometrical interferences.
To overcome the above-noted deficiencies, a sensor bracket or system of Stevenson, et al., U.S. Pat. No. 6,176,636 has been brought forth. Stevenson, et al. has a two-component bracket and drive washer combination which provides secure holding of a magnetic sensor while automatically setting an optimal air gap. Stevenson, et al. has a main bracket component having an aperture and a drive wall at one end of the aperture. A sensor body is connected to the main bracket component and is allowed to touch a target. A reaction bracket has a reaction wall in the aperture opposite the drive wall and is restrained from moving. A drive washer is provided with teeth having a pitch whereby as the washer is pressed between the drive and reaction walls along a transverse axis. The main bracket component is forced to move along a longitudinal axis, thereby moving the sensor body in relation to the target an amount precisely equal to a desired air gap. It is desirable to provide an arrangement for holding a sensor in a fixed position which is less expensive than that provided in Stevenson, et al. and wherein the bracket can possibly fabricated from a stamped sheet metal.
The present invention brings forth an arrangement for mounting a sensor at a fixed position with respect to a target. The arrangement includes a housing for a sensor. The arrangement also includes a mount spaced away from the target having a bore. A fastener is provided for insertion into the bore. The fastener has a shank with a tapered portion and also a head. A loop bracket is provided having an eyelet for encircling the housing. The bracket has at least partially overlapping opposite ends which have generally aligned holes for reception of the fastener shank. One of the brackets has a wedge tab. Upon insertion of the fastener shank into the bracket the wedge tab of the bracket makes contact with the tapered portion causing the bracket eyelet to close and capture the housing.
The present invention will now be explained by example with reference to the accompanying drawings.